This invention relates to lithographic printing. In particular, this invention relates to an aqueous developer for lithographic printing plates comprising a free radical polymerizable monomer and a radiation-sensitive initiator system.
In lithographic printing, ink receptive areas, known as image areas, are generated on a hydrophilic surface. When the surface is moistened with water and ink is applied, the hydrophilic regions retain the water and repel the ink, and the ink receptive areas accept the ink and repel the water. The ink is transferred to the surface of a material upon which the image is to be reproduced. Typically, the ink is first transferred to an intermediate blanket, which in turn transfers the ink to the surface of the material upon which the image is to be reproduced.
Lithographic printing plate precursors, typically called printing plates, typically comprise a radiation-sensitive coating applied over the hydrophilic surface of a support. If after exposure to radiation the exposed regions of the coating become soluble or dispersible in a developer and are removed in the developing process revealing the underlying hydrophilic surface of the support, the plate is called a positive-working printing plate. Conversely, if the exposed regions of the plate become insoluble in the developer, and the unexposed regions are removed by the developing process, the plate is called a negative-working plate. In each instance, the regions of the radiation-sensitive layer (i.e., the image areas) that remain are ink-receptive and the regions of the hydrophilic surface revealed by the developing process (i.e. the non-image areas) accept water and aqueous solutions, typically a fountain solution, and repel ink.
A common type of negative-working lithographic printing plate comprises a xe2x80x9cphotohardenablexe2x80x9d radiation-sensitive coating applied to a support. When the composition is exposed to radiation, the exposed regions are xe2x80x9chardenedxe2x80x9d (i.e., rendered less soluble in the developer) by the radiation. When the imaged plate is developed, the unexposed regions are removed and the exposed regions remain to form the image (i.e., the photosensitive composition is negative-working). In one, type of plate, the radiation-sensitive coating contains a composition that undergoes radiation-initiated free radical polymerization. These compositions comprise at least one multifunctional free radical polymerizable material such as a monomer and/or oligomer, a radiation-sensitive initiator system that forms free radicals on exposure to radiation, and, typically, a binder.
Free radical polymerization has been initiated by exposure with ultraviolet, with visible, and with infrared radiation. Such systems are described, for example, in: Baumann, U.S. Pat. No. 5,700,619 (ultraviolet); Ali, U.S. Pat. No. 4,971,8922 (visible); and Kojima, U.S. Pat. No. 5,550,002 (infrared). To prevent oxygen inhibition of the free radical polymerization, these plates typically have an overcoat that functions as a barrier layer.
Developers that can be used to develop these printing plates have been described in, for example, Walls, U.S. Pat. No. 5,035,982 [EP 482,098] and Joerg, U.S. Pat. No. 5,155,012 [EP 432,451]. However, when these developers are used, throughput (area of exposed printing plates developed, in m2, per liter of developer) is limited due to formation of sludge. During the development process, the unpolymerized free radical polymerizable material and the radiation-sensitive initiator system are removed from the unexposed regions of the plate by the developer. At least part of this sludge is formed by polymerization of some of the unpolymerized free radical polymerizable material in the developer, a process that is mainly initiated by interaction between the radiation-sensitive initiator system and the ambient radiation. Sludge is deposited on the rollers of the processor, which transfers it to both the image and non-image regions of the plate during development, adversely affecting the image produced by the printing plate. Thus, a need exists for an aqueous developer composition that develops these plates quickly, with high throughput, but with no sludge formation in the developing process.
In one embodiment, the invention is a composition for developing an imageable element comprising a negative-working composition that comprises a free radical polymerizable material and an initiator system. The method comprises the steps of:
(A) providing an imagewise exposed imageable element, in which:
the imageable element comprises a radiation-sensitive layer over a support,
the radiation-sensitive layer comprises a radiation-sensitive initiator system and at least one free radical polymerizable material, and
the radiation-sensitive layer comprises exposed regions and unexposed regions;
(B) applying a developer to the imagewise exposed imageable element, removing the unexposed regions of the imageable element, and forming the image in which:
the developer has a pH of about 5.0 to about 12.0, and
the developer comprises:
water,
a buffer, and
a component selected from the group consisting of (a) an added filter dye or mixture of filter dyes, and (b) an added free radical inhibitor or mixture of free radical inhibitors.
In one embodiment, the developer comprises about 75 wt % to about 98 wt % of water, a buffer, and about 0.1 to 2 wt % of the filter dye or mixture of filter dyes, or about 0.1 to about 3.0 wt % of the free radical inhibitor or mixture of free radical inhibitors. The filter dye or mixture of filter dyes and/or the free radical inhibitor or mixture of free radical inhibitors may be present in the developer. The developer may additionally comprise a surfactant or a mixture of surfactants and/or an organic solvent or a mixture of organic solvents.
In another embodiment, the invention is a composition useful as a developer for imageable elements that comprise a radiation-sensitive initiator system and at least one free radical polymerizable material. The composition comprises:
about 75 wt % to about 98 wt % of water;
about 0.5 wt % to about 15 wt % of an organic solvent or mixture of organic solvents;
about 0.5 wt % to about 15 wt % of a surfactant or mixture of surfactants;
a buffer, and
about 0.1 to 2 wt % of a filter dye or mixture of filter dyes, or about 0.1 to about 3.0 wt % of a free radical inhibitor or mixture of free radical inhibitors;
in which the composition has a pH of about 5.0 to about 12.0.